Honeycomb filters are in use in, for example, a filter for capturing fine particles present in an exhaust gas emitted from an internal combustion engine, a boiler or the like, particularly, diesel particulate.
The honeycomb filter used for such a purpose generally has, as shown in FIGS. 6(a) and 6(b), a structure having a large number of through-holes 3 extending in an X axis direction surrounded by partition walls 2, wherein each two adjacent through-holes 3 are plugged at an opposite end of the structure alternately so that each end face looks checkerboard pattern. In the honeycomb filter having such a structure, a fluid to be treated enters through-holes 3 which are not plugged at the inlet end face 42 of the structure, i.e. which are plugged at the outlet side end face 44; passes through porous partition walls 2; and is discharged from adjacent through-holes 3, i.e. those through-holes 3 plugged at the inlet side end face and not plugged at the outlet side end face 44. In this case, the partition wall 2 becomes a filter and, for example, soot emitted from a diesel engine is captured by the partition walls and deposited thereon. In the honeycomb filters used in such a manner, the sharp temperature change of exhaust gas and the local heating makes non-uniform the temperature distribution inside the honeycomb structure and there have been problems such as crack formation in honeycomb filter and the like. When the above honeycomb filter is used particularly as a filter for capturing a particulate matter present in an diesel engine, i.e. as a DPF, it is necessary to burn the carbon particles deposited on the filter to remove the particles and regenerate the filter and, in that case, high temperatures are generated locally in the filter; as a result, there have been problems such as reduction in regeneration efficiency due to non-uniformity of regeneration temperature and formation of cracks due to high thermal stress. Further, since the temperature distribution during regeneration is not uniform, it has been difficult to obtain the optimum temperature in the whole filter and achieve an improved regeneration efficiency.
Hence, there were proposed processes for producing a honeycomb filter by bonding a plurality of individual segments using an adhesive. In, for example, U.S. Pat. No. 4,335,783 is disclosed a process for producing a honeycomb filter, which comprises bonding a large number of honeycomb parts using a discontinuous adhesive. Also in JP-B-61-51240 is proposed a heat-shock resistant rotary regenerative heat exchanger which is formed by extrusion molding a matrix segment of honeycomb structure made of a ceramic material; firing them; making smooth, by processing, the outer peripheral portion of the fired segment; coating the bonding part of the resulting segment with a ceramic adhesive which turns to have substantially the same mineral composition as the matrix segment and a difference in thermal expansion coefficient of 0.1% or less at 800° C. after firing; and firing the coated segments. Also in the SAE paper 860008 of 1986 is disclosed a ceramic honeycomb structure obtained by bonding cordierite honeycomb segments with a cordierite cement. Further in JP-A-8-28246 is disclosed a ceramic honeycomb structure obtained by bonding honeycomb ceramic members with an elastic sealant made of at least a three-dimensionally intertwined inorganic fiber, an inorganic binder, an organic binder and inorganic particles. It was also tried to produce a honeycomb filter using, for example, a silicon carbide type material of high thermal conductivity and high heat resistance, in order to prevent its local heating to high temperatures and its damage due to thermal stress.
By using segment and/or a highly heat-resistant material such as silicon carbide type material, the damage caused by thermal stress can be prevented to some extent; however, it is impossible to eliminate the temperature difference between the outer peripheral portion and center of honeycomb filter, and uniform regeneration and consequent improvement in durability have been insufficient. Moreover, there have been cases of local heat generation during regeneration.
Also in JP-A-2001-162119 is disclosed a filter which is an integral ceramic filter structure produced using a sealant (adhesive) layer of 0.3 to 5 mm in thickness and 0.1 to 10 W/mk in thermal conductivity and in which the temperature of the whole part is made uniform and local unburned matter are hardly seen. By using an adhesive having a particular thickness and a particular thermal conductivity, it is possible to reduce local unburned matter and increase the efficiency of regeneration by burning soot; however, it is not sufficient in prevention of the temperature gradient during local heating to high temperature and the resulting thermal stress and has been insufficient in an increase in critical soot amount enabling regeneration by burning soot. Further, as disclosed in the above literature, by changing the thickness of the adhesive, it is possible to control the thermal conductivity and heat capacity of the adhesive; however, the increase in the thickness of the adhesive results in other inconveniences of reduction in effective area of filter and reduction in pressure loss characteristics when soot is deposited. Thus, the thermal conductivity and heat capacity and the pressure loss of filter become conflicting properties when it is intended to control them only by the thickness of the adhesive, and there is a limit in the thickness of sealant practically applicable in the filter.